How to add an extra CSS class to tickets buttons, using the Theater for WordPress plugin.

add_class_to_tickets_buttons.php

<?php
/**
 * Adds an 'art-button' CSS class to the tickets buttons of events.
 * 
 * @param 	array		$classes	The current CSS classes of tickets buttons.
 * @param 	WPT_Event	$event		The event that the button belongs to.
 * @return 	array					The new CSS classes.
 */
function add_class_to_tickets_buttons( $classes, $event ) {
	
	// Don't add anything when inside the WordPress admin.
	if ( is_admin() ) {
		return $classes;
	}
	
	$classes[] = 'art-button';
	return $classes;
}
 
add_filter( 'wpt/event/tickets/url/classes', 'add_class_to_tickets_buttons', 10, 2 );